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Spiral Galaxy NGC 3184
Kathy Gibes

NGC 3184

NGC 3184 is a spiral galaxy found about 12 Mpc (about 39 million light years) away. It has a diameter of 37kpc (about 119,000 light years). The Milky Way galaxy that we live in is also a spiral but it has a diameter of about 110,000 light years. So this spiral has a diameter slightly larger than the galaxy we live in. Spiral galaxies are much more difficult to form than elliptical galaxies. They need a quiet environment with very few collisions early in their life. The spiral arms can tell us many interesting things about the galaxy, one of which is the spin that it has. This galaxy is spinning clockwise.

Another thing we can learn from the spiral arms is the age of the stars in the galaxy. This galaxy appears blue because it has a lot of young, bright, blue stars. This is where most of the light is coming from in this galaxy. These stars began their life from the dust and gas that is located at the back of the arm, the inside of the curve. The reason why the stars form spiral arms is because of the idea of density wave. The stars that pass through these clouds of dust and gas get caught and slow down because it is much denser than where there is no arm. This galaxy has clumpy arms, which means that there are areas that have a higher concentration of new stars. There are also a few dimmer, red stars that are older. These stars are typically seen, in the optical, away from the arms. The reason why it seems that they are not along the arms is because that is where the very bright, new, blue stars are located. These stars's brightness overpower the dim stars; they are hidden by the blue stars.

NGC3184 Near Infrared

Figure 1: Near Infrared (1.2 micron) image of NGC 3184 taken by the 2MASS 1.3m telescope.

The image of NGC 3184, Figure 1, in the near infrared (2.2 micron) wavelength gives us an closer look at the F, G, K, M stars. These stars are older and redder than the bright blue stars. Since blue stars are so bright they over power the dimmer stars in the optical part of the spectrum. This is why this wavelength is so important. Also, this wavelength passes through dust without a problem, so we don't have to worry about dust extinction. There isn’t much dust in this galaxy but it goes through the dust that it does have. This is important because otherwise the dim F, G, K, M stars wouldn’t be as visible.

These stars are older so they have had time to travel away from the areas of star formation and are no longer in clumps, like the new, blue stars. This is why, in this wavelength, we can see a more even distribution of stars. You can slightly see the spiral arms because there is still a density wave along the spiral arms but they are not prominent. The reason for these arms being slightly visible is because these areas are denser so they slow the motion of the stars as they pass through.

In Figure 2, we can see two different wavelengths, 21 cm (shown as red) and the mid infrared (shown as green). These two wavelengths are superimposed on each other so it is difficult to see the full extent of the 21 cm emission.

Hydrogen spin flip

Figure 2: Demonstrating the Spin-Flip.

The red in Figure 3 shows where the neutral hydrogen (HI) is located in the galaxy. This means that the hydrogen is not in an excited state, they have very little energy. This gas extends almost to the center of the galaxy, so this boundary line is where the HI region begins (when starting at the center and moving out). In the center of the galaxy the neutral hydrogen has been compressed into molecules and will no longer go through the Spin-Flip (Figure 2). In every hydrogen, the proton and electron have their own spin and they both start spinning in the same direction. At this stage they are at a slightly higher energy level, but they prefer to be at the lowest possible energy level. To get to the lowest possible energy level, ground state, they interact with other particles and release some energy, in the form of a photon. This photon then travels to our telescope and this is what we see. When this occurs the electron will flip and spin in the opposite direction. The Spin-Flip is very rare but because of how many neutral hydrogen atoms there are in space it is occurring many times a second.

NGC3184 in radio and 8micron

Figure 3: NGC 3184 in the Radio part of the spectrum (21 cm - red) taken by the VLA, and 8 micrometer (green) from the Spitzer Space telescopes.

The green, 8 micron emission, is the glowing polycyclic aromatic hydrocarbons. When we look at a galaxy at this wavelength we can see these glowing because they are being warmed by the nearby, new, bright, warm stars. New stars form along the arms of the galaxy and this is why we can see the spiral arms decently. So, the main thing that we can learned from the 8 micron emission is where there is star formation. But, we can see that some of these molecules aren't along the arms. This is because the older, cooler stars will still be able to slightly warm them.

The pitch angle is a measurement of the tightness of a spiral galaxies spiral arms. An Sa spiral will have arms that are very tight to the center and Sc spirals have much looser arms. But NGC 3184 is an Sb galaxy. It's spiral arms are average. The pitch angle typically ranges from 5-30 degrees. But this is a unique galaxy and the spiral arms aren't opening at a constant angle.

Measuring the pitch angle

Figure 4: Calculations done on the spiral arms to determine the pitch angle.

The way you calculate the pitch angle is by first creating a circle with the core as the center. Then you create a tangent line to the circle where the spiral arm intersects it. Then you create a tangent line to the arm at the point of intersection. The pitch angle is the one between these two lines. You repeat this as many times as you like with a different diameter each time. I used four circles. The measurements I found for the two arms were (starting from the center) 34, 31, 25, 25 and 42, 31, 30, 28 degrees. When I did these measurements I found that the angle closest to the center was greater than the typical max of 30 degrees. But as I moved out along the arms the angle decreased. Both arms seemed to have a slight correlation; 42-34, 31-31, 30-25, 28-25. Of course there is human error, about 5 degrees, in these calculations so it could be that they are actually more symmetrical.

Another interesting thing about this galaxy is that it seems to have an extra arm branching off of one of the main arms. With simply looking at the image it is difficult to tell which branch would be more likely to be the main one, but the calculations make it clear. The angles coming off the branch are much larger (60 and 37) than they "should" be. Also, when you compare this to the unbranching arm the measurements don't match up well.


Brinks, Elias. "The Structure of Galaxies" Centre for Astrophysics Research, University of Hertfordshire. Accessed 19 April 2013. <>.

Kutner, Marc L. Astronomy: A Physical Perspective, 2nd ed. Cambridge: Cambridge University Press, 2003.

WolfWikis. "Spin-Flip Transition" Accessed 7 May 2013. <>

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Right Ascension (J2000) 10:18:19
Declination (J2000) +41:25:44
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter

7x300 seconds in B

4x300 seconds in V

2x300 seconds in R

7x300 seconds in C

Date observed

March 25, 2013 (BVRC)

To create the color image, I used images from each filter (blue, green, red, and clear). Before I was able to calibrate these images I had to remove the ones that had issues, such as a plane passing by. Then I calibrated the images using the dark, flat and bias images and created a single image for each of the four filters. Once I had these four images I combined them to create a color image with a R:G:B ratio of 0.8:1.07:5.4. When refining the brightness of my image I needed a gamma value of 0.6, minimum value of 5626, and a maximum value of 6496. I also had a few bad pixels in my galaxy image so I removed those. And to finish of my color image I changed the saturation to 185% and cropped the image so just the galaxy was visible.



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